Prior to the present invention, the general method for making silicone-styrene block polymers as shown by J. Saan et al, U.S. Pat. No. 3,678,125, involved the preparation of a block of polystyrene having terminal organometallic functional groups resulting from anionic polymerization. Silicone blocks were then grown off the ends of vinyl polymer by means of anionic ring opening of a cyclic siloxane such as hexamethylcyclotrisiloxane. The resulting ABA silicone-organic-silicone block polymer was then coupled to give a multiple sequence block polymer having good mechanical properties.
Although anionic polymerization can provide valuable multiple sequence block polymer, as further taught by J. Saam et al, anionic polymerization has a substantial drawback for making such materials since it requires rigorous exclusion of oxygen, water, as well as other impurities. Ultra-pure monomers must be employed to insure that the organometallic functional groups are preserved during polymerization. An additional disadvantage of anionic polymerization, is that a coupling reaction must be performed in order to obtain microstructures which have good mechanical properties. As a result, silicone-vinyl polymers have not been commercialized.
The present invention is based on the discovery that polyorganosiloxane polymers can be made having chemically combined free-radical precursor groups situated at regular intervals along the polymer backbone which on thermolysis serve as sites for free-radical initiation. As a result, the aforementioned functional oligomers are capable of further chain growth at the site of such chemically combined free radical precursor groups in the presence of vinyl monomers to produce silicone-organic block polymers.